Well fracturing operations are well known in the oil and natural gas drilling industries for increasing the flow capacity of a well. During a typical well fracturing operation, large amounts of abrasive and/or acidic fluids (slurries of sand, water, and various chemicals) are pumped down the well by high-pressure pumps. The high-pressure fluids (and sometimes gels) are intended to fracture the formation, thereby improving the permeability and flow capacity of the hydrocarbons. A frac head is typically connected to the wellhead (or above the wellhead). Multiple fluid lines connect the frac head to corresponding high-pressure pumps (typically pump trucks). The frac head acts as a manifold to collect and redirect fluid from the multiple fluid lines down through the well head into the well bore.
Owing to the abrasive (and sometimes corrosive) nature of the fracturing fluids, the interior bore of the frac head can be subject to extreme erosion. Such erosion is costly in that it can severely limit the useful service life of the frac head. The frac head wall typically needs to be sufficiently thick to support pressures up to about 15,000 psi, fluid velocities of 200 ft/min or more, and fluid flow rates of 100 gal/sec or more. Eroded frac heads can sometimes be repaired, but are often simply scrapped. Due to the high fluid pressures, frac head erosion also potentially poses a serious safety concern. Frac head ruptures are known in the art.
Numerous approaches have been taken to address frac head erosion. For example, frac heads have been fabricated from thick walled steel and/or with high-strength construction materials. The inner surface of the frac head has also been lined with various erosion resistant materials. Unfortunately these approaches have met with minimal success, most likely due in part to the extremely high pressures and fluid flow rates.
Recently, McLeod et al. (U.S. Pat. No. 6,899,172) disclosed a frac head utilizing an abrasion resistant wear sleeve deployed in the through bore of the frac head below the entry point of the side ports. The intent of the sleeve is to provide a replaceable, protective component that protects the side wall of the frac head from erosion. The sleeve is tapered (cone shaped) and held in place against a shoulder formed on the inner wall of the frac head. While the use of a replaceable sleeve may reduce erosion to the frac head side wall, it is not without drawbacks of its own. For example, removal of the sleeve can be difficult, often requiring a machine press or other time consuming operations. As stated in the McLeod patent: “the sleeve is held in place by friction, as small particles of sand fit between the sleeve in the bore of the frac head.” This friction is compounded by the high pressure fluids being forced through the tapered sleeve which can act to wedge the sleeve in place. The sleeve may become wedged in so tightly that removal thereof can damage the inner wall of the frac head.
It is also possible for the wear sleeve to become dislodged from its seat due to a surge of back pressure from the well. Such pressure surges are commonly encountered in fracturing operations. The dislodged sleeve may then block one or more of the top or side ports, which can render the tool inoperable for continued use. In such an event, removal and repair of the frac head would possibly be required prior to continuing the fracturing operation, resulting in a significant loss of rig time. Moreover, in applications in which a slick or wireline tool assembly is deployed above the frac head, a dislodged sleeve may cause the tool to become stuck in the well or even lost (due to severing of the cable).
Therefore, there is a need for an improved frac head. In particular, there is a need for a frac head that resists or is substantially free from internal erosion. Moreover, such a frac head should not require time consuming and costly reworking operations (e.g., reworking of the inner surface) or replacement of various components (such as wear sleeves).